Power transmissions employ a hydraulic fluid circuit to provide lubrication and hydraulic power to various components within the transmission, such as gears, clutches, brakes, etc. These transmissions therefore need a storage area for the hydraulic fluid that is not in transit during transmission operation.
The total hydraulic fluid within the transmission is equal to the transmission that is in the transmission sump, the transmission fluid that is currently stored within a component, such a servomechanism for a clutch or brake, and the hydraulic fluid that is in transit between the transmission pump and the sump.
It is desirable to make the transmission sump volume as small as possible in order to maintain a smaller overall package for the transmission. The volume of the sump oil is required to be large enough to keep the hydraulic pump primed under all conditions and to allow for some expansion for the oil under operating conditions wherein the oil temperature is increased or under elevated speed conditions wherein the volume of fluid that is in transit is decreased. It is desirable that the sump be large enough to prevent the oil level from being operated on by the rotating parts in the transmission. These rotating parts include drums, gears, and shafts, to name a few. The continuous operation of the rotating parts beneath the level of the sump oil reduces the overall efficiency of the transmission as well as causing foaming within the transmission fluid at elevated speeds. Both of these conditions are undesirable.
Prior art transmission structures have employed external reservoirs to store excess fluid and/or thermally controlled reservoirs, which provide for fluid storage at elevated temperatures or permit the fluid to return to the basic sump when the temperature decreases, such as when the transmission sits for a long period of time without operation. Both of these additional sump or reservoir systems require valving and control mechanisms so that the main sump is always provided with a minimum amount of oil.